1. Field of the Invention
The present invention relates to a propeller for a boat, including a variable-diameter blade or variable-pitch blade mounted to a propeller boss connected to a propeller shaft, and an exhaust passage defined in the propeller boss for discharging an exhaust gas from an engine.
2. Description of the Prior Art
The boat propellers conventionally known include a stationary blade type having all of blades integrally formed on the propeller boss, and a variable blade type having all of blades pivotally supported such that their pitches or propeller diameter can be variable (for example, see Japanese Patent Application Laid-open No. 144287/90). In the propeller, it is also known that the exhaust passage is defined in the propeller boss to extend axially through the propeller boss in order to discharge the exhaust gas from the engine driving the propeller into water by utilizing the propeller.
When the exhaust passage is formed in the propeller boss, a sufficiently large sectional area of the exhaust passage can be insured in the propeller of the stationary blade type without any obstruction by the blades. In the propeller of the variable blade type, however, the exhaust passage is interfered by the blade shaft supporting the variable blades or the like and necessarily constricted, resulting in an increased resistance to the exhaust gas and a reduced output from the engine.
Accordingly, it is an object of the present invention to provide a propeller for a boat, wherein the variable-diameter blade or the variable-pitch blade are provided on propeller boss, but an exhaust passage of a large passage area can be formed in the propeller boss.